Weddings are supposed to be joyous family occasions. There is always a sense of happiness for the newly wedded couple as well as a positive and hopeful outlook on the new couple's future together. To put it bluntly, Mr. Hooper's black veil ruins all of those feelings.
What makes it worse is that the text tells readers that Mr. Hooper has always previously been a bright and cheery person at weddings. He is the kind of person whose very presence calms and encourages the nervous couple while at the same time conveying a general cheerfulness to all the wedding's attendees:
That night, the handsomest couple in Milford village were to be joined in wedlock. Though reckoned a melancholy man, Mr. Hooper had a placid cheerfulness for such occasions, which often excited a sympathetic smile where livelier merriment would have been thrown away. There was no quality of his disposition which made him more beloved than this.
When Mr. Hooper shows up at the wedding, he does not usher in the standard happy wedding feelings. Instead, readers are told that Mr. Hooper brings in an overall sense of evil:
When Mr. Hooper came, the first thing that their eyes rested on was the same horrible black veil, which had added deeper gloom to the funeral, and could portend nothing but evil to the wedding.
We are told that the people in attendance feel as if the room has grown darker. The bride becomes so pale that people think that she looks like the dead body from the day's earlier funeral. We are specifically told that Mr. Hooper's black veil creates one of the most "dismal" weddings ever.